.box {
    overflow: hidden;
    background-color: aqua;
    margin: 0 auto;
    width: 1000px;
    height: 1000px;
}

.inner {
    margin: 20px auto;
    background-color: saddlebrown;
    width: 500px;
    height: 500px;
    border: 20px solid salmon;
    box-sizing: border-box;
}